Most commonly abdominal pain that occurs only at bedtime is related to not wanting to go to bed. This may be due to not wanting to separate from you ( separation anxiety or fear of the dark ) or because he knows going to bed means waking up and going to school in the morning. Ask him gently about his fears and heck with his teacher about his school days: friends, bullying, trouble understanding. As you get him to open up, you should be able to figure out the issues behind the complaints.

There are many reasons for this, but almost none of them relate to sleep disorders. I would start by speaking to your pediatrician.